Systems are known in which it is a requirement to transmit data from a first device to a second device, and from the second device to the first device, over a single transmission line. Thus, in half-duplex systems, there are periods during which the first device is able to transmit data, and periods during which the second device is able to transmit data.
In such systems, it is necessary to be able to transmit data signals from the first device to the second device, and to be able to detect the signals in the second device, and to be able to transmit data signals from the second device to the first device, and to detect the signals in the first device.
In such systems it may be desirable to ensure that the transmission lines are terminated with an impedance matching the characteristic impedance of the transmission line to reduce reflections of the signal on either end of the transmission line.
U.S. Pat. No. 5,781,028 describes a Universal Serial Bus (USB) interface, with respective terminations coupled to the two ends of a data cable, with the possibility to selectively activate the terminations. For a USB bus, the switching of transmission direction is relatively infrequent, but for other applications such as bidirectional transmission of digital audio streams the direction of transmission may be desired to be much more frequent to meet targets of signal latency. Each reversal of direction requires careful synchronisation and timing margin for the switching activity to cope with various possible transmission distances during which the bus can not transmit useful data, and requires power to switch large low-resistance switches. Also, each reversal of direction may cause Electromagnetic Interference emissions.